Because of the toughness and flame retardancy, cellulose acylate films have been traditionally used for photographic supports and various optical materials. Particularly, in recent years, cellulose acylate films are frequently used as optical transparent films for liquid crystal display devices. Cellulose acylate films exhibit high optical transparency and high optical isotropy and thus, are excellent as the optical materials for devices dealing with polarization, such as liquid crystal display devices. Therefore, cellulose acylate films have been used heretofore as polarizer protective films, or as supports for optical compensatory films whereby display viewed from a tilted direction can be improved (viewing angle compensation).
A polarizer, which is one of the elements constituting a liquid crystal display device, is constructed by bonding a polarizer protective film to at least one side of a polarizer. A polarizer in general can be obtained by staining a stretched polyvinyl alcohol (PVA) film with iodine or a dichromatic dye.
In many cases, as the polarizer protective film, cellulose acylate films, in particular, triacetyl cellulose films, which can be directly bonded to PVA are used. Such a polarizer protective film should be excellent in optical isotropy, and the characteristics of a polarizer greatly depend on the optical characteristics of the polarizer protective film.
Recent liquid crystal display devices are more strongly demanded for improvements in viewing angle characteristics. Thus, optical transparent films such as polarizer protective films or retardation films (also called optical compensatory films) are required to be more optically isotropic. For an optical film to be optically isotropic, it is important that the retardation value of the film, which is represented by the product of the birefringence and thickness of the film, is small. Especially, in order to improve the display viewed from a tilted direction, it is necessary to reduce the retardation in the front direction (Re) as well as the retardation in the thickness direction (Rth). More specifically, it is needed that upon evaluation of the optical properties of an optical transparent film, Re measured at the film front is small, and Re does not change even though the measurement angle is varied.
To the present, there have been cellulose acylate films having reduced front Re values, but it has been difficult to produce cellulose acylate films having little change of Re with the angle, that is, having small Rth. Thus, there have been suggested optical transparent films having reduced angle change for Re by using polycarbonate films or thermoplastic cycloolefin films instead of cellulose acylate films (for example, JP-A Nos. 2001-318233 and 2002-328233, products available as ZEONOR (Zeon Corp. in Japan), ARTON (JSR Corp.), etc.). However, when used as polarizer protective films, these optical transparent films have a problem in bonding to PVA because the films are hydrophobic. There is another problem that the overall in-plane optical properties of the films are non-uniform.
As a solution to these problems, there has been a strong demand for a further improved cellulose acylate film, which attains excellent suitability for bonding to PVA by lowering the optical anisotropy. More specifically, the solution is an optically isotropic optical transparent film obtained by lowering the frontal Re of the cellulose acylate film to almost zero, and reducing the change in retardation angle, that is, lowering the Rth to almost zero.
In the production of a cellulose acylate film, generally a compound called a plasticizer is added to improve the film-forming performance. Known examples of the plasticizer include phosphoric acid triesters such as triphenyl phosphate and biphenyldiphenyl phosphate, and phthalic acid esters (See, for example, Lecture on Plastic Materials, Vol. 17, Nikkan Kogyo Shinbun, Ltd., “Cellulose Resins”, p. 121 (1970)). It is known that some of these plasticizers have an effect of lowering the optical anisotropy of cellulose acylate films, and for example, specific fatty acid esters are known (See, for example, JP-A No. 2001-247717). However, it cannot be said that the effect of lowering the optical anisotropy of cellulose acylate films using these conventionally known compounds is sufficient.
Furthermore, recent liquid crystal display devices are also increasingly demanded to have improved display colors. Therefore, optical transparent films such as polarizer protective films or supports for optical compensatory film are required to have smaller Re or Rth in the visible light region with the wavelength ranging from 400 to 800 nm, as well as smaller changes in Re or Rth dependent on the wavelength, that is, smaller chromatic dispersion.
In general, a liquid crystal display device (LCD) comprises a liquid crystal cell and polarizers. The polarizer comprises protective films and a polarizing film, and is obtained by staining a polarizing film made of a polyvinyl alcohol film with iodine, stretching the polarizing film, and laminating protective films on both sides of the polarizing film. In a transmissive liquid crystal display device, this polarizer is mounted on both sides of the liquid crystal cell, and one or more of optical compensatory sheets may be also disposed. In a reflective liquid crystal display device, a reflective plate, a liquid crystal cell, one or more optical compensatory sheets and a polarizer are disposed in the same order. The liquid crystal cell comprises liquid crystalline molecules, two sheets of substrates to enclose the molecules in between, and an electrode layer to apply voltage to the liquid crystalline molecules. The liquid crystal cell performs ON/OFF display on the basis of the difference in the orientation state of the liquid crystalline molecules, and there have been suggested display modes such as TN (twisted nematic), IPS (in-plane switching), OCB (optically compensatory bend), VA (vertically aligned) and ECB (electrically controlled birefringence), that are applicable both to the transmissive and reflective types.
Among these LCDs, for the uses where high display quality is needed, a 90° twisted nematic liquid crystal display device (hereinafter, referred to as TN mode) which employs nematic liquid crystal molecules having positive dielectric anisotropy and is operated by a thin film transistor, is mainly used. However, the TN mode shows such viewing angle characteristics that although the devices have excellent display characteristics when viewed from the front, their display characteristics deteriorate when viewed from a tilted direction, in a manner such that the contrast is reduced, or there occurs tone reversal in which brightness is reversed in gray-scale display, or the like. Thus, improvements in these characteristics are highly demanded.
In recent years, as a form of LCD having improvements in such viewing angle characteristics, there has been suggested a vertically aligned nematic liquid crystal display device (hereinafter, referred to as VA mode), in which nematic liquid crystal molecules having negative dielectric anisotropy are employed such that the longer axes of the liquid crystal molecules are approximately aligned in a vertical direction on the substrate, without any voltage applied, and these liquid crystal molecules are operated by a thin film transistor (See JP-A No. 11-258605) This VA mode shows excellent display characteristics at a level equivalent to the TN mode when viewed from the front, and also exhibits expanded viewing angle characteristics due to the application of a retardation film for viewing angle compensation.
It is also known that in the VA mode, even expanded viewing angle characteristics can be realized by using a uniaxially oriented retardation film having positive refractive index anisotropy and a negative uniaxial retardation film having the optical axis in a direction perpendicular to the film plane (See JP-A No. 11-258605).
However, in such a VA mode liquid crystal display device, if the optical anisotropy of the transparent protective film (also accomplishing the role as a support) holding the retardation films and the polarizers is not designed to have an appropriate value, satisfactory viewing angle characteristics cannot be obtained. Furthermore, when the PVA film constituting the polarizer contracts under the effect of heat and humidity, stress is exerted on the transparent protective film holding the polarizer, whereby a change occurs in the optical anisotropy of the protective film itself. Thus, there have been failures, in particular, deteriorating the display quality of so-called corner spots, which is light leakage at four corners of the screen.
Thus, a first object of the present invention is to provide a liquid crystal display device having excellent display characteristics and viewing angle characteristics.
In addition, it is also known for the VA mode that characteristics of wider viewing angle can be obtained by using two sheets of negative uniaxial retardation films having the optical axes in a direction perpendicular to the film plane, above and below the liquid crystal cell, and that characteristics of expanded viewing angle can be realized by using uniaxially oriented retardation films having positive refractive index anisotropy and having an in-plane retardation value of 50 nm in this LCD (See SID 97 DIGEST pp. 845-848).
However, use of two sheets of retardation films (See SID 97 DIGEST pp. 845-848) is associated with an increase in the production costs, and has problems that bonding of a plurality of films causes lowering in the process yield, use of a plurality of films causes increased thickness, and it is disadvantages for the thickness reduction of the display device. Furthermore, use of an adhesive layer for laminating stretched films causes contraction of the adhesive layer under changes in temperature and humidity, and thereby defects of delamination or warpage of films may occur. As methods for improving these problems, there are disclosed a method of reducing the number of retardation films (See JP-A No. 11-95208) and a method of using a cholesteric liquid crystal layer (See JP-A Nos. 2003-15134 and 11-95208). However, even these methods necessitate bonding of a plurality of films, and thus are unsatisfactory from the viewpoints of thickness reduction and production costs reduction. Moreover, there have been problems that light leakage in a tilted direction of the polarizer during dark state is not completely suppressed in the visible light region, and the viewing angle is not sufficiently expanded. A more important point is that, complete compensation of light leakage is difficult for the entire range of wavelength of the visible light with respect to the incident light in a tilted direction of the polarizer during dark state, and accordingly there is a problem that azimuthal direction-dependence of color shift occurs. There is also suggested a method of preventing light leakage by controlling chromatic dispersion of the retardation film (See JP-A No. 2002-221622), but there are problems that the difference between chromatic dispersion of the in-plane retardation and chromatic dispersion of the retardation in the thickness direction is not taken into account, and the effect of inhibiting light leakage is insufficient. Furthermore, the effect of changing the birefringence of the liquid crystal layer is not sufficiently considered, and a sufficient effect cannot be obtained, depending on the birefringence value of the liquid crystal layer.
Therefore, a second object of the invention is to provide a liquid crystal display device, particularly a VA mode liquid crystal display device, having high contrast due to accurate optical compensation of the liquid crystal cell. In particular, it is an object of the invention to provide a liquid crystal display device, particularly a VA mode liquid crystal display device, having reduced light leakage in a tilted direction during dark state and thus having improved viewing angle contrast.
Further, for example, Japanese Patent No. 2587398 describes a technique of expanding the viewing angle by applying an optical compensatory sheet, which is produced as discotic liquid crystals are applied on a triacetyl cellulose film, aligned and fixed, to a TN mode liquid crystal cell. However, with regard to liquid crystal display devices to be used for television sets, which are assumed to be viewed from various angles through large screens, it is difficult to require viewing angle dependence therefrom, and it is impossible to satisfy the requirement even with the method described above. Therefore, liquid crystal display devices that are different from the TN mode, such as IPS (In-Plane Switching) mode, OCB (Optically Compensatory Bend) mode and VA (Vertically Aligned) mode, are being investigated. Particularly, the VA mode is attracting interest as a liquid crystal display device for TV, since it has high contrast and a relatively high process yield in the production of large size screens.
As an optical compensatory sheet for the VA mode, there are known a compensatory sheet having a film (plate A) having an in-plane optical axis as an optically positive single axis and a film (plate C) having an optical axis in the normal direction of the film as an optically negative single axis, and a compensatory sheet which can realize viewing angle expansion by using an optically biaxial film (See JP-A Nos. 11-258605 and 11-133413). However, these literatures do not mention about improvements in color shift when the viewing angle is tilted from the front during dark state. In such optical compensatory sheet for the VA mode, the color shift in the case of tilting the viewing angle from the front during dark state can be improved by taking the chromatic dispersion of Re(λ) as a reciprocal chromatic dispersion (Re(λ) in the shorter wavelength side is smaller than Re(λ) in the longer wavelength side), and taking the chromatic dispersion of Rth(λ) as a chromatic dispersion proper (Reλ in the shorter wavelength side is larger than Re(λ) in the longer wavelength side), and an optically biaxial film having such chromatic dispersion characteristics is disclosed (WO 2004/068226 A1).
As a specific embodiment of the optical compensatory sheet for the VA mode, JP-A No. 11-95208 discloses an optical compensatory sheet in which a vertically uniaxially stretched film of a norbornene polymer (trade name: ARTON) and a film in which a layer containing an organic viscous composite (trade name: Lucentite STN, Lucentite SPN) in a hydrophobic resin (trade name: Denkabutyral #3000-K) is formed on a triacetyl cellulose film (trade name: FUJITACK SH-80). However, the Re(λ) chromatic dispersion of the vertically uniaxially stretched film of ARTON is not dependent on the wavelength while being uniform, and the Rth(λ) chromatic dispersion of the triacetyl cellulose film is a reciprocal wavelength distribution. Thus, from the viewpoint of improving the color shift in the case of tilting the viewing angle from the front during dark state, the performance is unsatisfactory. Furthermore, the optically anisotropic layer containing the organic viscous complex in the hydrophobic resin is less durable and soft, and thus the processability of the layer is not good.
As a specific embodiment of the optical compensatory sheet for the VA mode, JP-A No. 2002-311243 describes an optical compensatory sheet prepared from an optically anisotropic layer comprising rod-shaped nematic liquid crystals on triacetyl cellulose (trade name: FUJITACK T-50 SH) and another optically anisotropic layer comprising cholesteric liquid crystals; an optical compensatory sheet prepared from an optically anisotropic layer consisting of a polyimide layer on the triacetyl cellulose and another rod-shaped nematic liquid crystal layer; and a polarizer including an optical compensatory layer, having either of the above-described optical compensatory sheets laminated on a polarizer which employs triacetyl cellulose as a protective film. Even for any one of these two polarizers including optical compensatory sheet, the Re(λ) chromatic dispersion of the optically anisotropic layer comprising rod-shaped nematic liquid crystals is chromatic dispersion proper, and the Rth(λ) chromatic dispersion of the triacetyl cellulose film is reciprocal chromatic dispersion. Thus, from the viewpoints of improving the color shift in the case of tilting the viewing angle from the front during dark state, the performance is unsatisfactory.
Thus, a third object of the invention is to provide an optical compensatory sheet in which ideal chromatic dispersion characteristics capable of improving the color shift in the case of tilting the viewing angle from the front during dark state of a liquid crystal display device, are realized by lamination of a plurality of retardation films; a polarizer employing the optical compensatory sheet as protective films; and a liquid crystal display device employing the polarizer.